The present invention relates to an image forming apparatus such as copier, laser beam printer, etc., and more specifically to an image forming apparatus that forms images using at least a black toner.
The conventional image forming apparatus using the electro-photographic method is described here referring to FIG. 10. FIG. 10 is an outline configuration diagram of an image forming apparatus using the conventional electro-photographic method. The image forming apparatus is a full-color electro-photographic image forming apparatus having four photoreceptor drums and an intermediate transfer member. The image forming apparatus is provided with four image forming sections 10Y, 10M, 10C, and 10K, which are configured to have, in the neighborhood of a photoreceptor drum 1Y for yellow color, a photoreceptor drum 1M for magenta color, a photoreceptor drum 1C for cyan color, and a photoreceptor drum 1K for black color, chargers 2Y, 2M, 2C, and 2K, exposure units 3Y, 3M, 3C, and 3K, developing units 4Y, 4M, 4C, and 4K, and cleaners 8Y, 8M, 8C, and 8K, and the images formed on the photoreceptor drums in each image forming section are successively transferred by the transfer unit onto the belt shaped intermediate transfer member 6 that is adjacent to and passed over the photoreceptor drums, and the image transferred onto the intermediate transfer member 6 is further transferred in a second transfer section onto a recording material P such as paper sheet etc.
In the present image forming apparatus, the image density may change due to changes in the environment, or due to changes with the passage of time, or due to changes in the characteristics of the transfer rollers or of the intermediate transfer member used; or due to changes in the physical characteristics of the toner, or due to changes in the characteristics of the photoreceptor, and in general, image forming apparatuses are provided with mechanisms for adjusting the image density, most of which have a means that automatically make the image density appropriate. In particular, in an image forming apparatus giving outputs of full color images, in order to obtain the desired color balance, more accurate control is being demanded for each of the colors yellow, magenta, cyan, and black.
The conventional control of image density is explained here. Toner patch images of colors other than black are formed on the image carriers, toner patch images are formed on the intermediate transfer member by transferring from the image carriers and their optical densities are detected, and also a toner patch image of the black color is formed on the image forming body, and the image density is being controlled by detecting the optical density of the toner patch image on this image forming body (Patent Document 1).
Further, toner patch images of color toners and black toner are formed on image carriers, the optical density of the toner patch image transferred from the image carriers onto the intermediate transfer member is detected, and the transfer output is being obtained based on this optical density and the developing bias value transfer output (Patent Document 2).
Patent Document 1: Japanese Unexamined Patent Application Open to Public Inspection No. 2003-215888.
Patent Document 2: Japanese Unexamined Patent Application Open to Public Inspection No. 2003-15371.
However, the following problems are present in the background technology described above.
1) In Patent Document 1, although the density of the black toner patch image on the photoreceptor is being detected, the density sensor output is particularly low in the case of a solid black toner patch image and it is difficult to obtain the optical density with a good accuracy.
2) In Patent Document 2, although the density of the black toner patch image on the image carrier is being detected, the density sensor output is particularly low in the case of a solid black toner patch image and it is difficult to obtain the optical density with a good accuracy, and further, although the relationship between the developing bias voltage and the transfer output is obtained beforehand and the transfer output is being obtained based on the developing bias voltage, it is particularly difficult to obtain accurately the black toner transfer output in cases when the characteristics of the transfer roller or of the intermediate transfer member changes due to changes in the environment or due to passage of time.